In-process tape bur monitoring

ABSTRACT

The protective tape applied to the front surface of a semiconductor wafer to protect the wafer during backgrinding must be trimmed so that excess tape, known as tape bur, does not extend beyond the perimeter of the wafer. Tape bur may interfere with backgrinding by causing improper grinding, which may lead to wafer breakage. The tape cutter that trims the protective tape to eliminate tape bur is provided with a sensor which detects whether tape bur has been trimmed from the wafer. If tape bur has not been removed from the wafer, corrective action is taken to prevent the wafer from being backgrinded.

FIELD OF THE INVENTION

[0001] The present invention relates generally to backgrindingsemiconductor wafers, and more particularly to preventing excess tapebur from damaging the wafer during backgrinding.

BACKGROUND OF THE INVENTION

[0002] An exemplary semiconductor wafer 100 is shown in FIG. 1A. Thewafer 100 is formed by slicing a thin circular disk from a purifiedblock of silicon. The wafer 100 may have a thickness ranging from 500 to1000 microns. A typical wafer 100 may have a thickness of approximately740 microns. An integrated circuit fabrication process may be used toform a plurality of semiconductor dies 101 upon the wafer 100. FIG. 1Bis a cross sectional view of the wafer 100 and shows that the dies 101are located near the front surface 103 of the wafer. After the dies 101have been formed, a sawing process is typically used to separate thedies 101 from the wafer 100. Since the dies 101 are separated portionsof the wafer 100, the dies 101 have the same thickness as the wafer 100(e.g., 740 microns).

[0003] Each die 101 may be mounted within a package 150 to form asemiconductor chip 200, as shown in FIG. 2. The package 150, which isdesigned to protect the die 101 and to coupled a plurality of leads 125to the die 101, is only compatible with dies 101 having a specifiedrange of thickness. For example, a commercial die package may bedesigned to accommodate dies having a thickness of approximately 305microns. It is often desirable to reduce the thickness of thesemiconductor package. One method of reducing the thickness of a packageis to use a thinner package, which often requires the use of thinnerdies 101. For example, some packages may only be compatible withsignificantly thinner (e.g., 100 micron thick) dies 101. Thus, bothconventional and thin profile packages often require dies 101 which aremuch thinner than most wafers 100.

[0004] The procedure to reduce the thickness of the dies 101 so thatthey are compatible with a given package design is known asbackgrinding. This procedure takes advantage of the fact that the dies101 are formed near the front surface 103 of the wafer. Thus, the backsurface 104 of the wafer may be ground down to reduce the thickness ofthe wafer 100, and the dies 101 formed thereon, if the mechanical stressassociated with backgrinding can be controlled to avoid fracturing thewafer 100 or damaging the dies 101. FIG. 3 is a block diagram of abackgrinder 402, which includes a chucktable 300 and grinding wheel 310.The chucktable 300 is used to flatly support the wafer 100 as it isbackgrinded by the grinding wheel 310. The flat support offered by thechucltable 300 distributes stress induced by the grinding wheel 310,thereby reducing the chance of wafer fracture. Additionally, referringalso to FIG. 5, a layer of protective tape 320 is attached to the frontsurface 103 of the wafer 100 by a tape applicator 400. Thus, the layerof protective tape 320 lies between the front surface 103 of the wafer100 and the chucktable 300, thereby protecting the dies 101 and furtherabsorbing mechanical stress.

[0005] However, as illustrated by FIG. 4, when excess tape 325 extendssignificantly beyond the perimeter of the wafer 100, that excess tape325, also known as tape bur 325, may become trapped and folded betweenthe chucktable 300 and the wafer 100. The tape bur 325 can prevent thechucktable 300 from flatly supporting the wafer 100, and increase thepossibility of uneven back side grinding and possible wafer fracture.

[0006] In order to minimize this problem, the wafers 100 are processedby a tape cutter 401 before they are backgrinded. The tape cutter 401 isresponsible for trimming the protective tape 320 at or near theperimeter of the wafer 100, thereby removing any tape bur 325. However,the tape cutter 401 cannot consistently guarantee that each processedwafer 100 is free of tape bur 325 because the tape cutter 401 issusceptible to several malfunctions. Thus, under certain circumstances,a tape cutter 401 may output a wafer 100 with tape bur 325 forsubsequent processing by the backgrinder 402. Accordingly, there is aneed for an apparatus and a method to prevent a tape cutter fromoutputting a wafer with tape bur for subsequent processing by abackgrinder.

SUMMARY OF THE INVENTION

[0007] The present invention is directed at a method and apparatus forpreventing wafer breakage, caused the presence of a tape bur, during awafer thinning process. In the present invention, a sensor is added tothe tape cutting unit. The sensor enables the tape cutting unit todetect whether the protective tape has been properly trimmed. If thesensor concludes that the protective tape has not been properly trimmed,the tape cutter unit can take corrective action to prevent a waferhaving the improperly trimmed tape from being processed by thebackgrinder. For example, the sensor can trigger a circuit which haltsthe tape cutter. Alternatively, the circuit can cause the tape cutter toroute the improperly trimmed wafer to an alternate area, where it can beexamined by a technician or reprocessed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The forgoing and other advantages and features of the inventionwill be more clearly understood from the following detailed descriptionof the invention which is provided in connection with the accompanyingdrawings.

[0009]FIG. 1A is a top view of a semiconductor wafer containing aplurality of dies;

[0010]FIG. 1B is a cross sectional view of the semiconductor waferillustrated in FIG. 1A;

[0011]FIG. 2 illustrates a semiconductor die and package;

[0012]FIG. 3 illustrates a backgrinder containing a wafer having a layerof protective tape attached to its front surface supported by abackgrinder chucktable;

[0013]FIG. 4 illustrates how tape bur can prevent the chucktable of abackgrinder from flatly supporting a wafer;

[0014]FIG. 5 is a block diagram of a system used for backgrinding;

[0015]FIGS. 6A and 6B are illustrations of a portion of a prior art tapecutting unit; and

[0016]FIGS. 7A and 7B are illustrations of a portion of a tape cuttingunit in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Now referring to the drawings, where like reference numeralsdesignate like elements, there is shown in FIG. 5 a block diagram of asystem for backgrinding wafers. A wafer 100 is processed by a tapeapplicator 400, which attaches a protective tape 320 to the frontsurface 103 of the wafer 100. Excess tape surrounding the wafer, knownas tape bur 325, is trimmed at or near the perimeter of the wafer 100 bya tape cutter 401. The wafer 100 is subsequently processed by thebackgrinder 402, which grinds the back surface 104 of the wafer 100until the wafer 100 has been ground down to the desired thickness.Although FIG. 5 illustrates the tape applicator 400, the tape cutter401, and the backgrinder 402 as separate devices, it should beappreciated these devices 400-402 may be partially or fully integratedinto a lesser number of devices. For example, the tape applicator 400may be combined with the tape cutter 401, or the tape cutter 401 may beintegrated with the backgrinder 402.

[0018]FIG. 6A and 6B are more detailed diagrams of a prior art tapecutter 401. FIG. 6A is a top view while FIG. 6B is a side view. The tapecutter 401 accepts a wafer 100 including a layer of protective tape 320applied to the front surface 103 of the wafer 100. The tape cutter 401includes a cutting element 410 and an overload sensor 412. The cuttingelement may be any tool capable of cutting the protective tape 320. Forexample, the cutting element 410 could be a heater blade. The cuttingelement 410 is positioned at or a predetermined short distance beyondthe perimeter of the wafer 100 and may be rotated about the wafer 100 inorder to remove tape bur 325. Alternatively, the cutting element 410 maybe stationary while the wafer 100 is rotated. The overload sensor 412measures the stress encountered by the cutting element 410 and isdesigned to shut off or otherwise disengage the cutting element 410 ifunusual stress is encountered. After the wafer 100 has been processed bythe tape cutter 410, the wafer 100 is made available to the backgrinder402.

[0019] Unfortunately, the tape cutter 401 is incapable of consistentlysupplying wafers 100 to the backgrinder 402 which are free of tape bur325. A wafer 100 which has been processed by the tape cutter 401 mayhave tape bur 325 due to a several reasons. For example, the cuttingelement 410 may malfunction and fail to cut the tape 320. Alternatively,the cutting element 410 may not be aligned properly and might not cutthe tape 320 at the proper location. Unusual stress may have beenmeasured by the overload sensor 412 thereby causing the cutting elementto be turned off or disengaged. Since the tape cutter 401 always outputsits processed wafers 100 to the backgrinder 402, there is a possibilitythat a wafer 100 with tape bur 325 may be given to the backgrinder 402.

[0020] The tape cutter 401′ according to the present invention is shownin FIG. 7A (top view) and 7B (side view). The tape cutter 401′ issimilar to the prior art tape cutter 401, but includes a sensor 420which is coupled to a circuit 430. In the exemplary embodiment, thesensor 420 is a mechanical sensor which is placed behind the cuttingelement 410 relative to the direction A of cutting at a predeterminedshort distance (e.g., 0.5 mm) from the perimeter of the wafer 100. Theshort distance is chosen based on how much tape can protrude from theperimeter of the wafer 100 without increasing the risk of improper waferbackgrinding due to a tape bur. Thus, the sensor 420 may be placed atvarying distances from the perimeter based upon, for example, thethickness or the stiffness of the protective tape 320. Alternatively,the sensor 420 may be any other type of sensor capable ofdifferentiating between the presence and absence of the protective tape.For example, the sensor could be an optical sensor.

[0021] The sensor 420, which is coupled to a circuit 430 and which isprovided behind the cutting element 410 relative to the direction A ofcutting, is used to determine whether the cutting element 410 properlyand completely removed the tape bur 325 during a tape cutting operation.If the sensor 420 detects a tape bur 325 after the cutting operation,the circuit 430 causes the tape cutter 401′ to take corrective action.The corrective action can be any action which prevents the wafer 100with tape bur 325 from being processed by the backgrinder 402. In theexemplary embodiment, the circuit 430 halts the tape cutter 401′ andprevents automated movement of the wafer 100 to the backgrinder 402.Alternatively, the tape cutter 401′ could be triggered by sensor 420 andassociated circuit 430 to route improperly trimmed wafers 100 to analternate location (e.g., a reprocessing area). It is also possible tostop operation of the backgrinder 402 by a signal from circuit 430 whenthe backgrinder 402 receives a wafer containing a tape bur 325.

[0022] While certain embodiments of the invention have been describedand illustrated above, the invention is not limited to these specificembodiments as numerous modifications, changes and substitutions ofequivalent elements can be made without departing from the spirit andscope of the invention. Accordingly, the scope of the present inventionis not to be considered as limited by the specifics of the particularstructures which have been described and illustrated, but is onlylimited by the scope of the appended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. An cutting apparatus for removing a portion ofa protective tape from a wafer, comprising: a support for holding awafer having a protective tape thereon; a cutting element placed at afirst predetermined distance from said support for moving relative tosaid support to cut protective tape from a wafer placed on the support;a sensor for detecting if protective tape on a wafer is properly removedby said cutting element; and a circuit for initiating corrective actionwhen the sensor detects that a protective tape is not properly removedfrom a wafer by said cutting element.
 2. The apparatus of claim 1,wherein the circuit for initiating corrective action stops operation ofsaid cutting apparatus.
 3. The apparatus of claim 1, wherein the circuitfor initiating corrective action prevents a wafer on said support frombeing moved to a grinding area.
 4. The apparatus of claim 1, wherein thecircuit for initiating corrective action prevents a backgrindingapparatus from grinding the wafer.
 5. The apparatus of claim 1, whereinthe sensor is an mechanical sensor.
 6. The apparatus of claim 1, whereinthe sensor is an optical sensor.
 7. The apparatus of claim 1, whereinsaid first predetermined distance is approximately 0.5 mm from the edgeof a wafer placed on said support.
 8. The apparatus of claim 1, whereinthe sensor is placed behind the cutting element in a direction ofcutting action of said cutting element.
 9. A method for removing aportion of a protective tape from a semiconductor wafer comprising:cutting the protective tape at a predetermined distance from theperimeter of the wafer; sensing whether said cutting has properlyremoved the protective tape; and taking a corrective action if saidprotective tape has not been properly removed.
 10. The method of claim9, wherein the step of sensing further comprises sensing whether saidprotective tape exists at said predetermined distance from the perimeterof the wafer.
 11. The method of claim 9, wherein the corrective actionis halting further cutting operation.
 12. The method of claim 9, whereinthe corrective action is preventing the wafer from being subsequentlybackgrinded.
 13. The method of claim 9, wherein the step of sensing is amechanical sensing.
 14. The method of claim 9, wherein the step ofsensing is an optical sensing.
 15. The method of claim 9, wherein saidpredetermined short distance is approximately 0.5 mm.